Recent advances in microwave imaging have enabled commercial development of microwave imaging systems that are capable of generating two-dimensional and even three-dimensional microwave images of objects and other items of interest (e.g., human subjects). At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors (hereinafter referred to as “antenna elements”) to capture either passive microwave radiation emitted by the person or other object or reflected microwave radiation reflected from the person or other object in response to active microwave illumination of the target. A two-dimensional or three-dimensional image of the person or other object is constructed by scanning the array of antenna elements with respect to the object's position and/or adjusting the frequency (or wavelength) of the microwave radiation being transmitted or detected.
Microwave imaging systems typically include transmit, receive and/or reflect antenna arrays for transmitting, receiving and/or reflecting microwave radiation to/from the object. Such antenna arrays can be constructed using traditional analog phased arrays or binary reflector arrays. In either case, the antenna array typically directs a beam of microwave radiation containing a number of individual microwave rays towards a point or area/volume in 3D space, hereinafter referred to as a target. The target can be associated with the object and correspond to a voxel or a plurality of voxels in an image of the object or the target can be a microwave receiver for capturing microwave radiation reflected off the object. This is accomplished by programming each of the antenna elements in the array with a respective phase-shift that allows the antenna element to modify the phase of a respective one of the microwave rays. The phase-shift of each antenna element is selected to cause all of the individual microwave rays from each of the antenna elements to arrive at the target substantially in-phase. Examples of programmable antenna arrays are described in U.S. patent application Ser. No. 10/997,442, entitled “A Device for Reflecting Electromagnetic Radiation,” issued as U.S. Pat. No. 7,224,314; and patent application Ser. No. 10/997,583, entitled “Broadband Binary Phased Antenna.”
As a result, for each target, each antenna element is programmed with a particular phase-shift to produce a beam of microwave radiation that experiences the highest constructive interference at the target. The combination of all the phase-shifts assigned to the antenna elements in the array for a particular target is referred to as a pattern. The size of the pattern is the same size as the array, and each element in the pattern represents the phase-shift of a corresponding antenna element in the array. In the case of a binary array, where each antenna element can introduce only one of two phase-shifts, the pattern can be represented as an array of ones and zeros.
When using reflector antenna arrays that are designed to both reflect microwave radiation from a microwave source to a target associated with the object and reflect microwave radiation reflected from the object target to a microwave receiver target, the patterns for each reflection (to the object-target and to the receiver-target) may contain conflicting values. For example, a particular antenna element in the reflector array may have a 0 degree phase-shift in a transmit design to reflect the microwave radiation from the microwave source to the object-target and a 180 degree phase-shift in a receive design to reflect the microwave radiation from the object-target to the microwave receiver-target.
To capture a microwave image of the target-object, substantially simultaneous reflection between the microwave source and the microwave receiver is required. Therefore, in order to use a single reflector antenna array for both transmission and reception of the microwave radiation, each antenna element can be programmed with only one phase-shift value. If the two phase-shift values for a particular antenna element in the transmit and receive patterns contradict each other, only one of the phase-shift values can be used for that particular antenna element during image capture. What is needed is a method to determine the phase-shift value for each antenna element when incorporating two patterns (transmit and receive) onto the array.